Organic amine absorbents are commonly utilized in heat reversible absorption processes to remove acid gases such as sulfur oxides (SO.sub.2 and SO.sub.3), hydrogen sulfide, carbon dioxide, carbonyl sulfide, and the like from gas streams, especially hydrocarbon gas streams. A thermal desorption step is typically used to regenerate the absorbent. Other acids present as a reaction byproduct, such as formic, sulfuric, thiocyanic, oxalic, hydrochloric, and the like, can complex with the amine to form heat stable organic amine salts (HSS) which are non-reversible under ordinary process desorption conditions. Such acids may be present in the gas as a minor component or may result from oxidation of the amine sorbent or the gas constituents. If not counteracted, the HSS impurity can increase in concentration and tie up available amine, impairing efficiency and increasing the corrosivity of the absorbent solution.
A caustic reclaimer unit may be suitable for regenerating certain organic amine sorbents such as monoethanolamine (MEA) from the HSS. The free MEA may be subsequently recovered from the caustic by distillation. Such reclaiming processes, however, are time consuming, not readily adaptable to on-site field operations and incur the costs of transporting spent absorbent solution to the site of the unit. Furthermore, amine sorbents other than MEA are generally heat sensitive and cannot be distilled following neutralization.
Organic amine sorbents can also be regenerated using limestone. The free amine is then separated from the solids by filtration. Limestone regeneration has defects similar to those of caustic reclaiming. In addition, filtration is messy and trace concentrations of CaOH in the process will cause precipitation and fouling in the absorber.
Alternatively, it is known that organic amine absorbents may be recovered from the HSS by electrodialysis action as in U.S. Pat. Nos. 4,814,051 and 4,808,284 to S. Bedell et. al. Both of these patents disclose the use of a simple two compartment electrodialysis cell in acid gas conditioning processes utilizing amines. In the cell, an electric field effects separation of the HSS anions across an anionic selective membrane into the anode compartment and neutralization of the cations in the cathode compartment.
U.S. Pat. No. 2,768,945 to Shapiro discloses a method for separating acidic gases from a fluid mixture by aqueous amine absorption. A partial electrolytic purification of the amine solution recycled from the regeneration step to the absorption step is said to prevent the accumulation of strong non-volatile acids.
Dialysis generally refers to processes wherein aqueous solutions of organic or inorganic acids or bases are produced by diffusion across a membrane of specific selectivity as disclosed, for example, in U.S. Pat. No. 4,818,409 by Puetter, et al. Electrodialysis is known in the art to include an applied electric field to effect diffusion. Donnan dialysis, on the other hand, is a non-electrodialysis process based on the Donnan exclusion principle wherein an ion exchange across the membrane occurs. Diffusion of a dialyzer ion species is concentration driven whereupon diffusion of an exchanging ion species is driven by the principles of electroneutrality and conservation of charge across the membrane.
The primary problem associated with dialysis separation is the difficulty of operating such processes efficiently. Mass transfer must be adequate for the species to be separated; otherwise, the required time and membrane area become unreasonably great. Also, the mass transfer rate for the product species must be low to avoid product losses into the neutralizing solution.
The use of anionic, cationic, or non-ionic (neutral) membranes to effect various dialysis processes are described in the RAIPORE Products Catalog distributed by RAI Research Corp. of New York. Dialysis is said to be useful in desalting applications, in reaction rate and pH control, in toxic anion or trace metal ion removal, in sensors, in bioreactors and in water treatment processes.
An electrodialysis process is described in a paper by K. N. Mani, available from Aquatech Systems of New Jersey, entitled "Electrodialysis Water Splitting Technology." Various anionic, cationic and bipolar membranes are disclosed to concentrate ions in solution and convert such ions into the acid or base. In this process, dialysis is driven by an electrical potential and water is split.
U.S. Pat. Nos. 4,543,169; 4,414,090; 4,230,549; 4,107,005; 3,674,669; 3,427,206; 4,468,441; 4,339,473; 4,113,922; 4,012,303; and 3,556,965 assigned to RAI Research Corp. disclose various aspects of the manufacture and use of dialysis membranes.
In commonly assigned U.S. Pat. No. 5,236,678, there is described a process for organic amine absorption of an acid gas such as SO.sub.2 by an aqueous solution of a compound represented by the formula: ##STR1## wherein each Y is independently --(CR.sup.2.sub.2)--, --(CR.sup.2).dbd. or --(C.dbd.O)-- and each R.sup.1 and R.sup.2 is independently hydrogen, an alkyl group; a hydroxyalkyl group; an aldehyde group; a carboxylic acid group or salt thereof; an alkyl group containing at least one carboxylic ester, a carboxylic acid or salt thereof, ether, aldehyde, ketone, or sulfoxide group; wherein the compound has at least one carbonyl group. Representative of such absorbents are 2-piperidone and ethyl piperidone 2-carboxylate.
In commonly assigned U.S. Pat. No. 5,098,681 there is described a process for organic amine absorption of an acid gas such as SO.sub.2 by an aqueous solution of a compound represented by the formula: ##STR2## wherein each R.sup.1 or R.sup.2 is independently hydrogen; an alkyl group; a hydroxyalkyl group; an aldehyde group; a carboxylic acid or salt group; an alkyl group containing at least one carboxylic ester, carboxylic acid or salt, ether, aldehyde, ketone or sulfoxide; and wherein at least R.sup.1 or R.sup.2 is a carbonyl-containing group, such as an aldehyde group, a carboxylic acid containing group, a carboxyl ester group, or a ketone-containing group; or ##STR3## wherein each R.sup.3 is independently hydrogen; an alkyl group; a hydroxyalkyl group; an aldehyde group; a carboxylic ester, a carboxylic acid or salt, ether, aldehyde, ketone or sulfoxide group. Representative of such absorbents are ethyl-1-piperazine carboxylate, 1,4-diformyl piperazine, 1-succinyl piperazine, glycine anhydride, and 1,4-dimethyl-2,5-piperazinedione.
In commonly assigned application Ser. No. 927,584, filed Aug. 10, 1992, now U.S. Pat. No. 5,342,593, entitled "Process for Absorption of Sulfur Compounds from Fluids" (applicants Steven H. Christiansen, Dane Chang and David A. Wilson), there is described a process for organic amine absorption of an acid gas such as SO.sub.2 by an aqueous solution of a compound represented by the formula: ##STR4## wherein X.sup.1 is --O--, --NR.sup.1 --, or --N.dbd.; each Y.sup.2 is independently --(CR.sup.2.sub.2)--, --(C.dbd.O)--, --O--, NR.sup.1 --, --N.dbd., or --C(R.sup.2).dbd.; each R.sup.1 and R.sup.2 is independently hydrogen; an alkyl group; a hydroxyalkyl group; an aldehyde group; a carboxylic acid or salt group; or an alkyl group containing an aldehyde group, a carboxylic acid or salt group, ketone, carboxylic ester, ether, or sulfoxide group; and m is an integer preferably of from 2 to about 4. Representative of such absorbents are hydantoin, N.sup.2,N.sup.3 -dimethyl-propyleneurea, trialkyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, and 2-oxazolidone.
In commonly assigned U.S. Pat. No. 5,108,723 there is described a process for organic amine absorption of an acid gas such as SO.sub.2 by an aqueous solution of a compound represented by the formula: ##STR5## wherein each R.sup.1, R.sup.2 or R.sup.3 is independently hydrogen; an alkyl group; a carboxylic acid group; a hydroxyalkyl group; an aldehyde group; or an alkyl group containing a carboxylic ester, a carboxylic acid or salt, ether, aldehyde, ketone or sulfoxide group; wherein at least one of R.sup.1 and R.sup.3 is a carboxymethyl group and at least one of R.sup.1 and R.sup.3 is hydrogen, preferably at least one R.sup.1 is carboxymethyl and at least one R3 is hydrogen. Representative of such absorbents is symmetrical ethylenediaminediacetic acid.